Recently, research has been conducted on reflective optical devices mainly with a view to detection and imaging of infrared rays. In particular, for instance, JP-2763055B, JP-2598501B, and JP-2716933B propose various optical devices in each of which reflection surfaces are eccentrically disposed so that light fluxes are not blocked by the reflection surfaces on their ways but are effectively directed and imaged. Furthermore, a reflective optical device in which a reflection surface is formed to be a free-form surface, though not for use with infrared rays, is proposed by JP8(1996)-292371A, for instance.
As an imaging optical system for imaging image information in a visible range, a refractive optical system utilizing an optical glass as a lens has been used. An optical device having a plurality of reflection surfaces outside an optical medium that is a transparent body is proposed by JP11(1999)-14906.
On the other hand, in an optical device imaging a thermal image by utilizing infrared rays, since it deals with optical rays with wavelengths in the infrared range, a material used in a lens is selected from the group consisting of germanium, silicon, ZnSe, etc. An optical device having a lens formed with germanium and ZnSe is proposed by JP10(1998)-339842A. An optical device having a lens formed with germanium and silicon is proposed by JP1(1989)-88414A.
On the contrary, a reflective optical system is characterized in the capability of image formation in a range from the visible range to the infrared range by appropriately selecting a material of a reflection surface. JP10(1998)-206986A proposes an optical device having a relay lens in addition to a coaxial reflective optical system with two reflection surfaces of a main mirror and a sub mirror.
A conventional optical system in which reflection surfaces are eccentrically arranged as described above, however, has a drawback in that optical specifications of brightness, resolution, distortion, an angle of view, etc. do not suffice for practical use.
Furthermore, in a conventional optical device for imaging image information in the visible range as described above, a refractive optical system requires a multiplicity of lenses for imaging image information in the visible range; thereby having a high cost. Furthermore, in an optical device in which reflection surfaces are provided outside a transparent optical medium, the number of component members required is decreased, but sufficient cost reduction cannot be achieved.
On the other hand, in the case where a refractive optical system is used in an optical device for forming a thermal image by utilizing infrared rays, the cost is high, and in some cases toxic materials such as germanium or ZnSe are used. In the case of a reflective optical system, the system can be made to have a relatively great numerical aperture without using specific materials, but reflection surfaces thereof per se tend to block light fluxes, thereby causing the optical system to have a small angle of view.